Remove Drupal::CORE_COMPATIBILITY because it is not accurate when modules can be compatible with multiple core branches

+1 to removing this constant. To get the ball rolling, here are all 7 of the occurrences of 'CORE_COMPAT' in 10.1.x branch of core.

Some initial thoughts on the above list...

1. core/includes/update.inc:          $ret[$module]['warning'] = '<em>' . $module . '</em> module cannot be updated. It contains an update numbered as ' . \Drupal::CORE_MINIMUM_SCHEMA_VERSION . ' which is reserved for the earliest installation of a module in Drupal ' . \Drupal::CORE_COMPATIBILITY . ', before any updates. In order to update <em>' . $module . '</em> module, you will need to install a version of the module with valid updates.';
   

   This is fully wrong. 😉 IMHO, it should be using \Drupal::MAJOR_VERSION (see 📌 Add \Drupal::getMajorVersion() Needs work ) not always printing '8.x' in this warning message.

   Also, shouldn't this message all be wrapped in t()?

2. core/lib/Drupal/Core/Extension/Dependency.php:      $this->constraint = new Constraint($this->constraintString, \Drupal::CORE_COMPATIBILITY);
   

   This one is more tricky to undo.

3. core/lib/Drupal/Core/Extension/ModuleDependencyMessageTrait.php:      $version = str_replace(\Drupal::CORE_COMPATIBILITY . '-', '', $modules[$dependency]->info['version'] ?? '');
   

   Here we're using it to conditionally strip off any '8.x-' from a contrib module's version string. Perhaps we don't need a constant for this and can hard-code it in here.

4. core/lib/Drupal/Component/Version/Constraint.php:   *   Normally this is set to \Drupal::CORE_COMPATIBILITY by the caller.
   

   This is the bulk of where we need to figure out what we want.

5. core/lib/Drupal.php:  const CORE_COMPATIBILITY = '8.x';
   

   To be removed. 🤞

6. core/modules/system/system.install:        $version = str_replace(\Drupal::CORE_COMPATIBILITY . '-', '', $required_file->info['version'] ?? '');
   

   Same story as ModuleDependencyMessageTrait... only using it to optionally strip '8.x-' from contrib versions.

   Perhaps we want to move some of this stripping logic into core/lib/Drupal/Component/Version somewhere so we can put the hard-coded weirdness in one spot and re-use it anywhere we're converting contrib bespoke version strings for comparisons.

   Or maybe core/lib/Drupal/Core/Extension/ExtensionVersion.php would be better for this... In fact, it's already got a CORE_PREFIX const in there.

Adding #3093789: system_requirements() strips core compatibility in really fragile and dangerous way when comparing versions → as related re: point 6 (and 3) above...

Adding a related issue for schema version.